1. Field of the Invention
The invention relates to ring antennas for resonant circuits, which may be used to identify objects, and to interrogation systems including such ring antennas. Such resonant circuits may be used in transponders, readers, or the like. In particular, the invention relates to annular antennas for resonant circuits, which may be mounted on and/or around metal containers, and to interrogation systems including resonant circuits and such annular antennas.
2. Description of the Related Art
Wireless systems have been developed for remote monitoring of various parameters. Such systems may be used to identify or detect the presence, location, and number of objects at a distance. For example, an interrogation apparatus or system may use a radio frequency (RF) signal to request information from a transponder, also known as a responder device or apparatus. This information may include a preset code for identification purposes, or the requested information may be based on measurements from a sensor.
Transponders may include electronic or integrated circuits and resonant circuits used for receiving RF signals from an interrogation device or apparatus and for transmitting RF signals containing monitoring information back to an interrogation device or apparatus. Such resonant circuits include a capacitance, an inductance, and a resistance. Further, energy from the signals received by the resonant circuit may be stored and used to power the resonant circuit, thus eliminating the need for power supplies in many applications. Such stored power also may be used to excite the resonant circuit for transmission of signals back to the interrogation device or apparatus.
To further conserve energy, resonant circuits have been designed, which need not be continuously excited. Instead, the circuit may be initially excited and then periodically supplied with additional energy to maintain oscillation. This maintenance energy is supplied because the resonant circuits may suffer various losses. Other components of the transponder also may cause a damping of the oscillation, thus, resulting in decay of the oscillation. The re-excitation of a resonant circuit with maintenance energy pulses is referred to as "plucking."
With known transponders, the plucking function may be initiated after a fixed number of oscillations. For example, the plucking function may be initiated after every eight oscillations (also referred to as "ringings") of the resonant circuit. Unfortunately, the quality-factor, i.e., the Q factor, of resonant circuits is not constant from transponder to transponder, and thus, the frequency of the plucking function may require modifications for proper operation. For example, a plucking function that is performed every eight oscillations, i.e., 8-pluck, may be insufficient to maintain oscillations in a low Q factor resonant circuit, e.g., a circuit with a Q factor in the range of about 10 to 40. Moreover, the Q factor of individual resonant circuits may fluctuate, due to the presence of metal or circuits used to dampen other nearby resonators.
Small, inexpensive, and highly reliable transponders are generally desirable. However, transponders may be used in a wide range of applications, and thus, different transponders are commonly designed for different applications. Various transponder embodiments are described in U.S. Pat. No. 5,053,774; which is incorporated herein by reference. Similarly, their associated antennas are designed to meet a variety of needs. The need for different designs causes transponder and antenna costs to increase and may cause their reliability to decrease.
Nevertheless, air coil transponders are sensitive to the close proximity of metal. Distances constituting close proximity are relative to the mass and the geometry of metal objects and to the particular type of metal from which the objects are made. The close proximity of metal tends to detune the antenna circuits, and metal, especially iron and alloys containing iron, tend to dampen the RF signal. Although ferrite rod transponders are generally less sensitive to the close proximity of metal when the rod is positioned parallel to the metal surface, field orientation is often not optimal in such antenna applications. In general, metal objects in a range of less than about 15 mm of the antenna of an air coil transponder are considered in close proximity, and metal objects in a range of between about 8 to 10 mm of the antenna of a ferrite rod transponder are deemed to be within close proximity. Pot core transponders represent an alternative configuration to air coil transponders, but these transponders may attain only limited read ranges due to size and design constraints.